1. Field of the Invention
This invention relates to methods of treatment and pharmaceutical compositions used for the prevention and treatment of corneal haze. The formation of corneal haze as an artifact resulting from exposure of the cornea to laser irradiation is a new phenomenon. The methods of the present invention for the prevention and treatment of corneal haze involve the application of compositions to the eye prior to, during and after irradiation.
2. Description of Related Art
Ophthalmic operations such as corneal transplants and keratotomies traditionally are performed by surgeons using cutting instruments. At the incision site the cutting edge of these instruments unavoidably damages several layers of cells on either side of the point of entry. This impairs the ability of the surgical wound to heal without resulting scar tissue. The use of lasers in ophthalmic surgery has developed partly in an effort to minimize damage to cells as a result of disturbance due to instrument incisions. Infrared lasers such as carbon dioxide (CO.sub.2) lasers have been used to achieve controlled local ablation or incision of the cornea. However, use of CO.sub.2 lasers and most lasers emitting in the visible spectrum, also damage or char cells adjacent to the site of ablation or incision because results are thermally achieved through photocoagulation and/or photovaporization.
Lasers emitting radiation at ultraviolet (UV) wavelengths do not have the same effect on adjacent cells as do infrared lasers or lasers which emit radiation in the visible spectrum. On impact, UV radiation decomposes the molecules of tissues photochemically by directly breaking intramolecular bonds. Thus, there is not the characteristic damage to adjacent cells resulting in photothermal ablation.
The use of UV lasers, which emit radiation below about 400 nanometers (nm), has dramatically improved the possibilities for ophthalmic surgery. For example, corneal transplants and keratotomies may be more precisely performed with less damage resulting from the procedure itself. Methods for conducting ophthalmic surgery using an ultraviolet emitting laser are described in U.S. Pat. Nos. 4,665,913 and 4,732,148 issued to L'Esperance, both of which are incorporated herein by reference to the extent that they disclose methods and devices for achieving a predetermined refractive correction by volumetric removal of corneal tissue. Removal is through the phenomenon of photoablation of the cornea namely, of the epithelium, Bowman's membrane and stroma levels of the cornea, using incident UV laser irradiation.
The ophthalmic surgical methods disclosed by L'Esperance include various procedures for correcting eye disorders attributable to abnormal curvature of the cornea, collectively referred to as laser refractive keratoplasties (LRK). It has been forecast that the eventual adoption of LRK to ablate the cornea for correction of common myopic hyperopic conditions will largely eliminate the need for eyeglasses, contact lenses or other methods of vision correction using lenses. LRK can also be used to correct astigmatisms; remove corneal scar tissue; and excise corneal tissue for accommodation of corneas in corneal transplants. In addition, LRK and other procedures involving lasers can be used to perform incisions, including incisions for refractive effects such as radial keratotomy.
Even with the improved surgical methods using UV emitting lasers, a condition known as "corneal haze" may result as a response to use of the laser during ophthalmic surgery. Corneal haze, as discussed herein, is an artifact which has not been observed as a result of ophthalmic surgery until the advent of use of the UV laser. The artifact is seen as opacification of the cornea, which in humans is composed of an epithelial layer, Bowman's membrane, the stroma, Descemet's membrane and the endothelium. The artifact resulting from UV laser surgery is seen in different parts of the cornea but particularly in the stroma. When the artifact does appear it can usually only be observed by use of a slit lamp. It is not known precisely why the artifact sometimes occurs after photoablation of the cornea. The development of corneal haze is of potentially greater concern in those procedures affecting a large surface of the cornea versus procedures involving laser incisions.
A study was done on the response of the corneal epithelium to excimer lasers (lasers based on the excited state of a halogen atom combining with the ground state of a rare gas such as krypton or xenon) due to concern over use of the lasers in work environments, for example, in isotope separation, thermonuclear fusion, photochemistry and underwater communications; see Taboada, et al., Health Physics, Volume 40, pp. 677-683 (May, 1981). In that study rabbits were exposed to pulsed laser radiation of 248 nm. The rabbits' corneal epitheliums exhibited increasing levels of damage with increasing exposure. As a result of this damage, the authors suggested that guidelines for workers exposed to short pulse radiation be redefined.
The corneal haze or artifact to be prevented or treated according to the present invention is not a result of the use of lasers in the work area. It is rather a result of purposeful and direct exposure of the cornea to UV irradiation during ophthalmic surgery. Recent studies have been done regarding the corneal haze which results from exposure to lasers. For example, in one study, seven patients received laser corneal ablation with a 193 nm Questek excimer laser. Post ablation examination with a slit beam showed a speckled haze at the interface between the epithelium and stroma; see Del Pero, et al., Human Excimer Laser Lamellar Refractive Keratotomy - A Clinical Study, ARVO Annual Meeting Abstract Issue, p. 281, No. 8 (1988). In another study ablation with a 193 nm excimer laser resulted in a slight haze in the corneas of rabbits and primates. The haze was observable by slit lamp but disappeared after two weeks. However, in the rabbits a material resembling plasma membrane was reported in Descemet's membrane, which is located between the posterior surface of the stroma and the anterior surface of the corneal endothelium; see Gaster et al., Excimer Laser Ablation and Wound Healing of Superficial Cornea in Rabbits and Primates, ARVO Annual Meeting Abstract Issue, p. 309, No. 4 (1988). In another study discs were formed in the corneal stromas of rabbits by excimer laser photoablation at 193 nm. A stromal haze developed by one month, but corneal transparency did improve after 6 months; see Tuft et al., Corneal Remodelling Following Anterior Keratectomy, ARVO Annual Meeting Abstract Issue, p. 310, No. 7 (1988).
With the improvement of lasers, particularly UV emitting lasers, and their use in ophthalmic surgery, there has developed a need for prevention of the corneal haze which results during ophthalmic procedures involving the use of lasers.
It is an objective of this invention to provide methods of treatment to prevent the appearance of and/or treat corneal haze induced by laser irradiation. It is another objective of this invention to provide compositions for treating eyes before, during or after exposure to a laser which will prevent and/or treat corneal haze.